1. Field Of The Invention
The present invention relates to a process for the preparation of dialkyl carbonates by reaction of carbon monoxide (CO) with alkyl nitrites in the presence of a catalyst from the series of platinum metal halides on a support from the series comprising oxidic or hydroxidic compounds of elements from group Vb of the Periodic Table (vanadium, niobium, tantalum), which catalyst may contain additives comprising compounds of further elements.
Dialkyl carbonates are of general chemical importance. Thus, for example, diethyl carbonate is an excellent solvent in the medium boiling range. Furthermore, dialkyl carbonates are excellent carbonylating and acylating reagents. Finally, they are of great importance in the preparation of other carbonates, of urethanes and of ureas.
2. Description Of The Related Art
The preparation of dialkyl carbonates by reactions of phosgene or alkyl chloroformates with alcohols is known. However, there is an increasing interest in replacing the use of the poisonous phosgene or the intermediates derived thereof, such as the chloroformic ester, by other processes. Apart from experiments to obtain dialkyl carbonates by reaction of CO with lower alcohols, in particular those processes are of importance in which CO are reacted in the gas phase with alkyl nitrite over a platinum metal catalyst. In reactions of this type, apart from the desired dialkyl carbonate, the formation of dialkyl oxalate is always observed.
Thus, EP 425 197 discloses a process which, according to its preferred embodiment, leads to dialkyl carbonates of methanol and ethanol from CO and methyl nitrite and ethyl nitrite in the gas phase over a PdCl.sub.2 catalyst on activated carbon. According to this EP 425 197 Table 1, the selectivities to give the desired lower dialkyl carbonates reach values of up to 94%; however, lower dialkyl oxalates and CO.sub.2 are always observed as by-products. Moreover, when this process was repeated, the high selectivities mentioned could only be reproduced insufficiently. The catalysts of this EP 425 197 contain additions of chlorides of base metals; a substantial addition of hydrogen chloride, i.e. an amount of 1 to 50 % mol, relative to the platinum metal in the catalyst, is introduced into the system, or a portion of the catalyst has to be removed from the reactor and subjected to a hydrogen chloride treatment.
In order to obtain dimethyl carbonate from CO and methyl nitrite, a carbon support is also used in Zeitschrift fur Katalytische Forschung (J. Catalytical Research, Dalian, China), Vol. 10 (1), pp. 75-78 (1989) as support for a PdCl.sub.2 -containing catalyst, which, however, also always leads to the additional formation of dimethyl oxalate.
A Pd/carbon catalyst is also mentioned in Chin. Sci. Bull. 34 (1989), 875-76 for the preparation of dimethyl carbonate from CO and methyl nitrite.
This preference for a carbon support is not unexpected, since Platinum Metals Review 34 (1990), 178-180 reports, with reference to old literature, that the reaction of a lower alkyl nitrite with CO over a Pd catalyst produces different main products, depending on the support; according to this report, a carbon support predominantly produces dialkyl carbonate, while an oxidic support, such as, for example, an Al.sub.2 O.sub.3 support mainly produces dialkyl oxalate.